This description relates to semiconductor devices and fabrication of semiconductor devices, and more specifically to methods of forming an ohmic contact on a thinned silicon carbide (SiC) semiconductor device and such devices.
Silicon Carbide (SiC) is a wide-band gap (WBG) semiconductor material that has material properties that make it suitable for high voltage, high power semiconductor devices. SiC substrates on which semiconductor devices are built start out approximately 350 μm thick. This thickness does not allow heat generated during device operation to dissipate fast enough to maintain predetermined operating temperatures. Therefore, the device is de-rated or auxiliary cooling is provided to ensure efficient device operation, which increases overall system level cost.
A primary advantage of SiC power electronic components is that very high power densities can be obtained in small footprint devices, enabling smaller, faster and more efficient power switching devices. A major challenge imposed by these high power density devices is the requirement to provide adequate thermal management in the packaged device. Without adequate thermal management, the power rating of the device falls far short of the intrinsic capability of the semiconductor. A key method for providing enhanced thermal management is to thin the semiconductor die after wafer processing on the device-side is complete to obtain a total thickness in the range of 75-200 μm.
Increasing wafer diameter leads to increasing thickness of the wafers for strength during processing. The contrasting interest of thinner dies for performance reasons makes wafer thinning techniques more and more important.
The formation of an ohmic contact (typically a silicide) on a back-side of a thinned SiC wafer, after the device-side processing has been completed, without damaging the device-side components poses challenges. Proper ohmic contact formation requires temperatures >1000° C. and is easy to complete when wafers are not thinned, as the temperature exposure is done early on in the process flow, before any die have structures or materials that can be damaged by high temperature because the melting point of some device-side materials begins around 600° C.
Laser annealing isolates high temperatures to a shallow region on the backs of thinned wafers, which prevents high temperatures from affecting vulnerable structures and/or materials. Controlling temperature such that the metal film needed to convert to silicide remains and is not ablated is a challenge for ohmic contact formation.
Laser anneal ablating causes some of the deposited films needed for silicide formation to be removed and no longer available for ohmic contact formation. Keeping the bulk film layer for complete silicide formation is a challenge with current laser anneal applications.